xen/acpi: upload Cx and Px data to Xen

When FreeBSD is running as dom0 (initial domain) on a Xen system it
has access to the native ACPI tables and is the OSPM. However the
hypervisor is the entity in charge of the CPU idle and frequency
states, and in order to perform this duty it requires information
found the ACPI dynamic tables that can only be parsed by the OSPM.

Introduce a new Xen specific ACPI driver to fetch the Processor
related information and upload it to Xen. Note that this driver needs
to take precedence over the generic ACPI CPU driver when running as
dom0, so downgrade the probe score of the native driver to
BUS_PROBE_DEFAULT in order for the Xen specific driver to use
BUS_PROBE_SPECIFIC.

Tested on an Intel NUC to successfully parse and upload both the Cx and
Px states to Xen.

Sponsored by: Citrix Systems R&D
Reviewed by: jhb kib
Differential revision: https://reviews.freebsd.org/D34841